Contamination investigation and risk assessment of molybdenum on an industrial site in China

Because certified reference standards on molybdenum (Mo) in soil are not available, and drafted risk assessment procedures in China have not included Mo. Assessment of the potential risks posed by Mo in soil and groundwater is the key to establishing the extent of the contamination and deriving achievable remedial targets, should remediation deemed to be necessary. This paper reports the first investigation of Mo contamination in soil and groundwater from an incandescent light-bulb manufacturing facility in China. Though the plant was built in 1971, the contamination of Mo in the site received little attention until the 1994 and 2008 monitoring campaigns; soil Mo concentrations ranged from not-detectable to 15 mg/kg in 1994, and from 0.25 to 252 mg/kg in 2008. In addition, groundwater Mo concentrations ranged from not-detectable to 362 μg/L in 1994, and 1 to 32, 500 μg/L in 2008. Simulation by Visual MINTEQ software showed that Mo speciation in the groundwater would be limited to MoO42 −. Detailed site investigation showed that the high concentrations of Mo in groundwater could be adequately explained by the predominant presence of anionic MoO42 − at the measured average soil pH of 8.65, and given the small adsorption coefficient 112 L/kg for Mo onto field soil samples. A modified three-step sequential extraction procedure showed that residual percentages of soil Mo at this industrial site ranged from 69.8 to 81.0%, indicating that Mo in the soil was mostly present in the mineral lattice. Bounding of Mo onto the mineral lattice is not available to living organisms and thus the risk from soil contamination was considered minimal.A conceptual site model developed for quantitative risk assessment indicated that the main exposure pathways would be consumption of the groundwater by local residents, and lateral Mo migration to bounding rivers. Only non-carcinogenic risk was assessed, because Mo has no known carcinogenic effect on living organisms, as indicated from the toxicity data for Mo. Among the 35 assessment locations, Mo in 14 locations was determined to pose unacceptable non-cancer risk for on-site children, with a hazard quotient from 1.2 to 433 when children directly drink groundwater. In addition, Mo in groundwater which is transported off-site by lateral migration will pose unacceptable non-cancer risks for off-site children, with a hazard quotient of up to 45.8 from direct water drinking. Although Mo concentrations in the bounding rivers ranged from 4 to 6,053 μg/L, toxicity data indicated that the ecological risk is minimal for aquatic biota in the surface water. A site-specific target level for Mo in groundwater was established as 75.1 μg/L. Further work will be conducted regarding remediation feasibility at this site; permeable reactive barriers may be an effective option given the predominant distribution of Mo in the groundwater in the site.